Gauge Invariant YFS Exponentiation of (Un)stable Z-Pair Production At and Beyond LEP2 Energies

We present the theoretical basis and sample Monte Carlo data for the YFS exponentiated calculation of ee → ZZ → f1f̄1 + f̄2f2 at and beyond LEP2 energies, where the left-handed part of fi is a component of an SU2L doublet, i = 1, 2. The calculation is performed for both SM couplings and for anomalous ZZV triple gauge boson couplings in the conventions of Hagiwara et al.. Our formulas, which are gauge invariant, are illustrated in a proto-typical YFS Monte Carlo event generator YFSZZ. To be submitted to Phys. Rev. D † Work partly supported by the Polish Government grant KBN 2P30225206 by the US Department of Energy Contracts DE-FG05-91ER40627 and DE-AC03-76ER00515. ⋆ On leave of absence from Institute of Computer Science, Jagiellonian University, Kraków, Poland. UTHEP-96-0702 July 1996 The problem of the precision calculation of the processes ee → ZZ + n(γ) → 4fermions + n(γ) at the higher energy range of LEP2 and at NLC (> 300 GeV) type energies is the subject of our paper. We are motivated to investigate this problem for a number of reasons, the most important of which is its role in connection with the verification and tests of the SU2L ×U1 model of Glashow, Salam and Weinberg [1] of the electroweak interaction. Indeed, at the NLC these and related processes are expected to play the primary role in such physics studies [2, 3] as precision tests of the fundamental non-Abelian triple and quartet gauge field self-interactions in principle, for example. In this paper, we present and illustrate, via sample Monte Carlo data, the rigorous YennieFrautschi-Suura (YFS) [4] exponentiated Monte Carlo approach [5] to these processes. The respective Monte Carlo event generator which realizes the calculation is exact in the infrared regime and is of leading logarithmic accuracy through O(α) in the initial state QED hard radiative regime. A higher precision realization of our methods and results is in progress [6]. We recall at this time the current state of the art insofar as Monte Carlo event generators for the processes ee → ZZ + n(γ) → 4fermions + n(γ) are concerned. In ref. [7, 8], both semi-analytical and Monte Carlo event generator results in the literature have been reviewed. What is new in our work in this paper is that we present for the first time the results of a calculation of theese processes in which all of the following are simultaneously featured: • realistic, finite pT , simulation of the respective multiple photon radiative effects on an event-by-event basis in which infrared singularities are cancelled to all orders in α • YFS exponentiated O(α) LL initial state radiation (ISR)